You can swap books here. I've belonged for about four years now and I love it.
http://www.paperbackswap.com/members/index.php
Not necessarily for the books that are posted there. I've got a wish list and occasionally something I've been wanting will pop up. But I have a lot of books posted there which get requested pretty regularly...then I can transfer the credits to their sister sites:
Swap a CD: Music Club offers Free CDs when you Trade, Swap or Exchange Used CDs
Swap a DVD: Movie Club offers Free DVDs when you Swap, Trade, or Exchange Used DVDs
Between the three sites I have been able to build up quite a nice collection of books, movies and music.
If you are wanting to buy and sell books, I don't know anymore. I started selling books on eBay in 1999 and it was quite a time. But around 2005-2006 eBay started making it difficult for small sellers, especially sellers of used media items such as books, music and movies to turn a profit. Since then they have changed their fee structure to try and draw these sellers back, but the buyers never seemed to return. It's a shame because ebay was truly remarkable at first. Just another example of how people in business think they are so clever (and greedy) and then fuck everything up.
I've never sold on Alibris or Amazon. I though about it but they are so impersonal for sellers, it just didn't motivate me. One of the other great things about selling books on ebay was the opportunity to interact directly with the 'book people' who were buying your books.
Anyway, sorry to go on and on. PaperbackSwap is a pretty cool way to swap books. And you can swap hardcovers and audio books there, too. Not just paperbacks.